As an exhaust gas purification device for purifying exhaust gas of a diesel engine, there is known an exhaust gas purification device which uses a DOC (Diesel Oxidation Catalyst) and a DPF (Diesel Particulate Filter) in combination.
In the exhaust gas purification device, soot in the exhaust gas is collected and discharged in the DPF. Since the DPF has the limit to the quantity of collected soot, DPF needs to be subjected to Active Regeneration by burning and removing the collected soot when the quantity of the collected soot in the DPF is within the limit quantity. The diesel oxidation catalyst provided at a stage prior to the DPF renders hydrocarbon (HC) and carbon monoxide (CO) in the exhaust gas harmless, and also has the function of oxidizing NO in the exhaust gas into NO2 to burn and remove the soot collected in the DPF, and the function of increasing the temperature of the exhaust gas by using oxidation reaction heat of an unburned component in the exhaust gas when the soot collected in the DPF is subjected to the Active Regeneration.
An example of means for increasing the temperature of the exhaust gas flowing into the DPF at the time of the Active Regeneration of the DPF includes means which supplies an unburned component to the DOC installed at a stage prior to DPF to utilize oxidation heat generated in the DOC.
Examples of a method for supplying the unburned component include Late Post injection which injects fuel at a timing when no contribution is made to combustion in a cylinder of a diesel engine, and a method which directly supplies fuel to an exhaust passage from an injection device installed in an exhaust gas passage.
Such method for supplying the unburned component is disclosed in, e.g., Patent Document 1.
[Patent Document 1] Japanese Patent Application Laid-open No. 2009-228589
However, when the fuel is supplied by the Late Post injection, a part of the fuel supplied into the cylinder of the engine is not sent to the exhaust gas passage, and is mixed into an engine oil in a oil pan in the lower portion of the engine. When the mixing ratio of the fuel in the engine oil is increased, there is a possibility that the lubricity of the engine oil is reduced, or the risk of ignition resulting from a reduction in flash point (oil dilution risk) occurs.